libclang: introduce cxstring::createNull()


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174173 91177308-0d34-0410-b5e6-96231b3b80d8
libclang: itroduce cxstring::createEmpty()


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174174 91177308-0d34-0410-b5e6-96231b3b80d8
Revise unit testing for -fno-altivec, -mno-altivec.

As suggested, reverted the end-to-end test and added variations to an
existing Driver test.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174176 91177308-0d34-0410-b5e6-96231b3b80d8